Solve for $a$ : $24 = -21 + a$
Solution: Add $21$ to both sides: $ \begin{eqnarray} \\ \hphantom{24 {+ 21}} &\hphantom{=}& \hphantom{a} \\ 24 &=& -21 + a \\ \\ {+21} && {+21} \\ \end{eqnarray} $ $ \begin{eqnarray} \\ 24 {+ 21} &=& a \\ \end{eqnarray} $ Simplify. $ a = 45$